Projection systems have been used for many years to project motion pictures and still photographs onto screens for viewing. More recently, presentations using multimedia projection systems have become popular for conducting sales demonstrations, business meetings, and classroom instruction.
In a common operating mode, multimedia projection systems receive analog video signals from a personal computer (“PC”). The video signals may represent still, partial-, or full-motion display images of a type rendered by the PC. The analog video signals are typically converted in the projection system into digital video signals that control a digitally driven image-forming device, such as a liquid crystal display (“LCD”) or a digital micro mirror device (“DMD”).
Significant effort has been invested into developing projectors producing bright, high-quality, color images. However, the optical performance of conventional projectors is often less than satisfactory. For example, suitable projected image brightness is difficult to achieve, especially when using compact portable color projectors in a well-lighted room.
Because LCD displays have significant light attenuation and triple path color light paths are heavy and bulky, portable multimedia projectors typically employ DMD displays in a single light path configuration. Producing a projected color image with this configuration typically requires projecting a frame sequential image through some form of sequential color modulator, such as a color wheel.
The use of color wheels in frame sequential color (“FSC”) display systems has been known for many years and was made famous (or infamous) in early attempts to develop color television sets. With technological advances, however, color wheel display implementations are still useful today.
FIG. 1 shows the operational concept of a typical prior art FSC display system 10 in which a sensor 12 senses a timing mark 14 to detect a predetermined color index position of a motor 16 that rotates a color wheel 18 having respective red (“R”), green (“G”), and blue (“B”) filter segments. A light source- 20 projects a light beam 22 through color wheel 18 and a relay lens 24 onto a display device 26, such as an LCD-based light valve or a DMD. A display controller (not shown) drives display device 26 with sequential R, G, and B image data that are timed to coincide with the propagation of light beam 22 through the respective R, G, and B filter segments of color wheel 18. Popular commercially available color wheel-based FSC multimedia projection systems are the LP300 series manufactured by In Focus Systems, Inc., of Wilsonville, Oreg., the assignee of this application.
To improve their projected image brightness, multimedia projection systems typically employ a high-intensity discharge (“HID”) arc lamp, which produces a point source of intense polychromatic light that is readily focused by a reflector onto a color wheel. HID arc lamps have many attributes, such as high intensity, efficiency, and reliability. Unfortunately, HID arc lamps provide more light at the blue end of the spectrum than at the red end. This leads to color balance problems, which prior workers attempted to solve in various ways including increasing the angular extent (physical size) of the color wheel R filter segment relative to the B filter segment and/or increasing the attenuation of the color wheel B filter segment relative to the R filter segment. Other workers tried reducing overall brightness levels through color lookup electronics to achieve “headroom” for color adjustments. Unfortunately these “solutions” either caused temporal artifacts or decreased image brightness.
Still other workers added a white (“W”) filter segment to the color wheel to provide a “white peaking” function, which does increase image brightness albeit at a loss of color saturation. Yet others have simply employed more powerful arc lamps, which in compact portable projectors, leads to heat, size, weight, cost, and reliability issues.
What is needed, therefore, is a multimedia projection system having an improved technique for achieving increased image brightness, color saturation, and adjustable color-balance.